1. Technical Field
The present invention relates to an optical fiber used for optical communication, and in particular to an optical fiber suitable for use as wiring of long distance lines for transmission over a length of tens of kilometers and as wiring in an optical fiber to the home (FTTH) or local area network (LAN) inside or outside of the home.
2. Related Art
Optical fiber is used in the field of long-distance communication, due to its bandwidth characteristics, and is widely used in communication via long-distance backbone cables with lengths of tens of kilometers or more. On the other hand, the amount of information exchanged between individual personal computers has increased drastically due to the quick spreading of the Internet. The communication paths that have become widely used are copper wire electrical cables, such as coaxial cables and unshielded twisted pair (UTP) cables. However, electrical cables have narrow bandwidth and are easily affected by electromagnetic noise, and it is therefore difficult to transfer a large amount of information through the electrical cables.
Accordingly, optical fiber is used not only for long distance communication between phone stations, but also for communication between phone stations and each user, and FTTH has become widely used as a technique for increasing the transmission capacity. The FTTH system utilizes the wideband characteristic of optical fiber to share a single optical fiber among a plurality of users at a point near a user group. After this, optical signals are branched to each user and optical fiber drop wires are distributed to each user.
Curvature loss is one important characteristic that is desired for optical fiber within home wiring or drop lines. Since a long distance backbone cable is arranged in a location that is not easily affected by outside forces, e.g. in an underground duct, the bending force placed on these optical fibers is expected to result from no more than wrapping the optical fiber (up to 100 times) with a diameter of 60 mm within a terminal device.
In contrast to this, although the optical fiber inside and outside a home can bend and is light-weight, the optical fiber is easily affected by outside forces and often experiences a curvature radius of 20 mm or less. The optical fiber propagates the signal light through the core of the optical fiber. Therefore, transmission is still possible when the optical fiber is in a curved state. However, when the curvature radius is smaller, the ratio of light that leaks out of the core without being propagated increases exponentially, resulting in transmission loss. This is referred to as “curvature loss.”
Focusing more of the light in the core is effective for reducing the curvature loss, and this effect can be improved by lowering the mode field diameter (MFD). Therefore, optical fiber with an MFD of approximately 6 to 8 μm is often used, in which case the curvature loss when the optical fiber is wrapped around a mandrel (cylinder) with a diameter of 20 mm is no greater than 0.5 dB/turn for a wavelength of 1550 nm.
A trench-type optical fiber that can lower the curvature loss while employing a design with high MFD is described in U.S. Pat. No. 4,852,968 and in the technical document “Optical Fiber Comprising a Refractive Index Trench” by William A. Reed. This technique has been known for a long time, but these excellent curvature loss characteristics have recently attracted a lot of attention. In the case of a quartz glass optical fiber, the core is doped with germanium to increase the refractive index and the trench portion is doped with fluorine to decrease the refractive index. Inner and outer cladding is formed by pure quartz or is doped with only a small amount of fluorine or germanium, thereby bringing the refractive index of the cladding near that of quartz.
When manufacturing a normal optical fiber base material using VAD, (1) a core (first core) and inner cladding (second core) are formed, to create a core/cladding glass intermediate body (intermediate body). Next, (2) the trench portion (third core) is formed. This is achieved by placing the intermediate body in a fluorine-doped glass tube, prepared separately. Instead, the trench portion can be formed by carefully blowing the outside of the intermediate body with glass soot microparticles, and thermally processing the resulting material in an atmosphere of a gas containing fluorine. Finally, (3) the outer cladding is formed.
Here, the problem is that impurities, such as OH groups, are easily mixed in near the interface between the intermediate body and the trench. When a large amount of OH groups are mixed in the path through which light travels in the optical fiber, an optical absorption peak occurs due to the OH groups at a wavelength of 1383 nm. As a result, the trench-type optical fiber manufactured in this way has high transmission loss at 1383 nm due to the OH groups, and it is difficult to satisfy the ITU-T G652D standard.
In light of the prior art above, it is an objective of the present invention to provide an optical fiber that has few OH impurities and excellent curvature characteristics. In particular, concerning the OH impurities, the transmission loss at 1383 nm is no greater than 0.35 dB/km, which is the transmission loss at a normal communication wavelength of 1310 nm.